Thin film optical coatings can be used to alter a substrate""s optical properties. For example, the reflection of light which occurs at the interface of two different materials may be altered by applying a thin film optical coating to a surface at such an interface. Additionally, the transmission of light can be reduced by an absorbent optical coating or the transmittance/absorbance of specific wavelengths can be enhanced.
It is often desirable to reduce the percentage of visible light which is reflected at an interface and increase the transmittance of visible light, thus reducing glare associated with the reflection of visible light. Antireflection thin film optical coatings for such purposes have numerous applications including, for example, windows, lenses, picture frames and visual display devices such as computer monitors, television screens, calculators and clock faces.
Generally, the reflection of light occurs at the interface of two materials which have different indices of refraction, for example, glass and air. Air has an index of refraction of approximately 1.00 and glass generally has an index of refraction of approximately 1.51, so that when light which was previously travelling through air becomes incident upon a glass surface, some of the light is refracted (bent) and travels through the glass at an angle different from the angle of incidence, and some of the light is reflected. Theoretically, in order to minimize the amount of light which is reflected from a glass surface, it would be ideal to coat the glass with a material having an index of refraction which is the square root of 1.51, which is the index of refraction of glass. However, there are very few durable materials which have such a specific index of refraction (i.e., 1.2288).
In order to overcome the problem created by the lack of durable materials having the requisite index of refraction, thin film coatings having multilayer designs have been developed. Prior multilayer antireflection coatings have included two, three, four and more layers. By using multilayer coatings with layers that have high, medium and low indices of refraction, in various combinations and orders, prior coating systems have been able to reduce the reflection of visible light at air/substrate interfaces to negligible percentages. However, each layer in a multi-layer coating system increases the overall cost of the coating system.
The are many different examples of multilayer coating systems that have previously been used. Two, three and four layer antireflection coatings are known and are described, for example, in H. A. Macleod, xe2x80x9cThin Film Optical Filters,xe2x80x9d Adam Hilger, Ltd., Bristol 1985. The coatings are designed to provide specific indices of refraction for different applications to deliver required optical properties. Indices of refraction are material constants. The index of refraction of a material, the amounts of a material, the combinations of materials and layer thicknesses all affect the optical properties of the resulting system. One such system commonly used is a xe2x80x9cthree-layer lowxe2x80x9d multilayer coating which has a medium index of refraction layer (xe2x80x9cM-layerxe2x80x9d) coated on the substrate, the M-layer having an index of refraction (xe2x80x9cnxe2x80x9d) of from 1.60 to 1.90, a high index of refraction layer (xe2x80x9cH-layerxe2x80x9d) coated on the M-layer, the H-layer having an n greater than 1.90, and a low index of refraction layer (xe2x80x9cL-layerxe2x80x9d) coated on the H-layer, the L-layer having an n less than 1.60, (thus providing an overall M/H/L structure). Other designs include bilayer coatings which generally have an M/L design which includes an inner M-layer and an outer L-layer. Such designs are useful, for example, with laser optic applications. Four layer systems are also known which generally have an H/L/H/L design and include an inner H-layer coated with an L-layer followed by a further H layer and L layer. Such coatings are typically used for technical applications which need to accommodate a somewhat greater amount of light passing through the coating then for standard applications.
Materials which are currently used in thin film optical coatings as layers having a high index of refraction include titanium oxide, hafnium oxide and other transition metal oxides. However, in order to produce durable coating layers of these high index of refraction materials, it is often necessary to use expensive techniques such as vacuum evaporation or sputtering. The cost of the equipment used in such application processes can often create an economically unviable approach to producing such coatings.
Other techniques by which layers of thin film optical coatings have been applied to substrates include the use of sol-gel technology. A common sol-gel technique includes the application of a solution to a substrate, with the subsequent conversion of an oxide precursor contained within the solution, to an oxide on the surface of the substrate. This method generally involves the removal of water by heat treatment. An alternative and more recently adapted technique of sol-gel chemistry involves the application of a colloidal suspension (sol) of a chemically converted oxide to a substrate with the subsequent evaporation of the suspending medium at room temperature. The first method is usually preferable due to the difficulties which may be encountered during the preparation of adequate colloidal suspensions.
The use of sol-gel chemistry in applying thin film optical coatings is desirable due to the prohibitive capital expenses associated with vacuum deposition equipment. Unfortunately, however, conventional sol-gel processes offer few choices of high refractive index coating materials.
Niobium oxide has been suggested for electrochromic applications, but thus far, it has not been used to produce a high index of refraction layer in thin film optical coatings, except through expensive sputtering and chemical vapor deposition techniques. Sol-gel techniques using niobium alkoxide precursors (such as niobium pentaethoxide, Nb(OCH2CH3)5) and niobium chloroalkoxide precursors (such as NbCl(OCH2CH3)4) have been used to create electrochromic coatings. Electrochromic coatings exhibit a reversible color change by alternating anodic and cathodic polarization. These coatings are usually spin-coated and generally have substantial thicknesses (5-10 xcexcm). Electrochromic materials are usually not very dense and are preferably amorphous to provide an open framework for rapid ionic diffusion. Electrochromic coatings are generally designed to be crack-free, but are not concerned with uniformity, or the absorption/transmission of light.
Niobium chloride and tetraalkoxysilane precursors have been used in combination in a molar ratio of 90:10 silicon to niobium as an L-layer material. Such precursor mixtures have produced materials with indices of refraction averaging approximately 1.55. It is generally well known and expected that combinations of two materials with differing indices of refraction will produce a material-mixture which has an index of refraction that is linearly and directly proportional to the molar ratio of the two components. For example, if one were to combine varying amounts of silicon dioxide and titanium dioxide (TiO2) and measure the index of refraction of the material-mixture as a function of the molar proportion of TiO2, a linear relationship would be observed. However, since precursor mixtures of silicon and niobium have been found to be unstable when niobium exceeds 10 mole %, these materials have not been heavily investigated. Precursors with greater than 10 mole % of niobium tend to undergo rapid gelation, rendering them ineffective for most sol-gel techniques.
While, sol-gel preparations have generally become a popular investigative topic in the field of thin film optical coatings, sol-gel niobium oxide materials are not known to have high indices of refraction.
When a sol-gel method is used to coat a substrate, the coating that is deposited generally requires a final heat cure to convert the coating into the desired oxide. A common cure temperature used in sol-gel applications is approximately 400xc2x0 C. There are many materials that have melting or decomposition points below 400xc2x0 C., including, for example, certain plastics and other polymeric resins. Thus, thin film optical coatings cannot be coated on a large class of materials (i.e., those with melting points below 400xc2x0 C.) using conventional sol-gel processes. Currently, heat-sensitive materials are coated by vacuum deposition.
Thus, there exists a need in the art for a durable material for use as a layer having a high index of refraction in a thin film optical coating which can be prepared in a relatively inexpensive manner. Additionally, inexpensive materials for use as layers having a medium index of refraction are also desired. Lastly, materials which are capable of providing high index of refraction layers on heat-sensitive materials are needed.
The present invention includes a thin film optical coating, having a layer comprising sol-gel derived niobium oxide, wherein the layer is capable of providing an index of refraction of at least about 1.90.
The present invention also includes a process for producing a thin film optical coating on a substrate, comprising: immersing the substrate in a mixture comprising niobium chloride and an alcohol; withdrawing the substrate from the mixture to provide the substrate with a coating of the mixture; and heat-treating the substrate to form a niobium oxide-based layer having an index of refraction of at least about 1.90.
The present invention also includes a thin film optical coating, having a layer comprising a sol-gel derived oxide system, the sol-gel derived oxide system comprising niobium oxide, silicon dioxide and aluminum oxide, wherein the layer is capable of providing an index of refraction of from about 1.60 to about 1.90.
The present invention further includes a process for producing a thin film optical coating on a substrate, comprising: immersing the substrate in a mixture comprising niobium chloride, a silicon precursor, an aluminum precursor, and an alcohol, wherein the molar ratio of niobium to silicon is from about 0.9:1 to about 3.6:1 and the molar ratio of niobium to aluminum is from about 0.8:1 to about 3.0:1; withdrawing the substrate from the mixture to provide the substrate with a coating of the mixture; and heat-treating the substrate to form a layer having an index of refraction of from about 1.60 to about 1.90.